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                            <div style="color:gray; word-break: break-all; font-size:12px;">原文地址: <a href="https://www.elastic.co/guide/cn/elasticsearch/guide/current/nested-objects.html" rel="nofollow">https://www.elastic.co/guide/cn/elasticsearch/guide/current/nested-objects.html</a>, 版权归 www.elastic.co 所有<br/>
                            英文版地址: <a href="https://www.elastic.co/guide/en/elasticsearch/guide/current/nested-objects.html" rel="nofollow">https://www.elastic.co/guide/en/elasticsearch/guide/current/nested-objects.html</a>
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<b>请注意:</b><br>本书基于 Elasticsearch 2.x 版本，有些内容可能已经过时。
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<h2 class="title">
<a id="nested-objects"></a>嵌套对象 (Nested Objects)<a class="edit_me edit_me_private" rel="nofollow" title="Editing on GitHub is available to Elastic" href="https://github.com/elasticsearch-cn/elasticsearch-definitive-guide/edit/cn/402_Nested/30_Nested_objects.asciidoc">edit</a>
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<p>由于在 Elasticsearch 中单个文档的增删改都是原子性操作，那么将相关实体数据都存储在同一文档中也就理所当然。比如说，我们可以将订单及其明细数据存储在一个文档中。又比如，我们可以将一篇博客文章的评论以一个 <code class="word">comments</code> 数组的形式和博客文章放在一起：</p>
<div class="pre_wrapper lang-json">
<pre class="programlisting prettyprint lang-json">PUT /my_index/blogpost/1
{
  "title": "Nest eggs",
  "body":  "Making your money work...",
  "tags":  [ "cash", "shares" ],
  "comments": [ <a id="CO267-1"></a><i class="conum" data-value="1"></i>
    {
      "name":    "John Smith",
      "comment": "Great article",
      "age":     28,
      "stars":   4,
      "date":    "2014-09-01"
    },
    {
      "name":    "Alice White",
      "comment": "More like this please",
      "age":     31,
      "stars":   5,
      "date":    "2014-10-22"
    }
  ]
}</pre>
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<div class="calloutlist">
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<p><a href="#CO267-1"><i class="conum" data-value="1"></i></a></p>
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<p>如果我们依赖<a class="xref" href="dynamic-mapping.html" title="动态映射">字段动态映射(dynamic mapping)</a>,那么 <code class="word">comments</code> 字段会自动映射为 <code class="literal">object</code> 类型。</p>
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<p>由于所有的信息都在一个文档中，当我们查询时就没有必要去联合文章和评论文档，查询效率就很高。</p>
<p>但是当我们使用如下查询时，上面的文档也会被当做是符合条件的结果：</p>
<div class="pre_wrapper lang-json">
<pre class="programlisting prettyprint lang-json">GET /_search
{
  "query": {
    "bool": {
      "must": [
        { "match": { "name": "Alice" }},
        { "match": { "age":  28      }} <a id="CO268-1"></a><i class="conum" data-value="1"></i>
      ]
    }
  }
}</pre>
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<div class="calloutlist">
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<p><a href="#CO268-1"><i class="conum" data-value="1"></i></a></p>
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<p>Alice实际是31岁，不是28!</p>
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<p>正如我们在 <a class="xref" href="complex-core-fields.html#object-arrays" title="内部对象数组">内部对象数组</a> 中讨论的一样，出现上面这种问题的原因是 <b>JSON 格式的文档被扁平化处理成简单的键值对结构</b>，比如：</p>
<div class="pre_wrapper lang-json">
<pre class="programlisting prettyprint lang-json">{
  "title":            [ eggs, nest ],
  "body":             [ making, money, work, your ],
  "tags":             [ cash, shares ],
  "comments.name":    [ alice, john, smith, white ],
  "comments.comment": [ article, great, like, more, please, this ],
  "comments.age":     [ 28, 31 ],
  "comments.stars":   [ 4, 5 ],
  "comments.date":    [ 2014-09-01, 2014-10-22 ]
}</pre>
</div>
<p><code class="word">Alice</code> 和 <code class="word">31</code> 、 <code class="word">John</code> 和 <code class="word">2014-09-01</code> 之间的相关性不复存在。虽然 <code class="literal">object</code> 类型 (参见 <a class="xref" href="complex-core-fields.html#inner-objects" title="多层级对象">多层级对象-内部对象</a>) 在存储 <em>单一对象</em> 时非常有用，但对于对象数组的搜索而言，毫无用处。</p>
<p><em>嵌套对象</em> 就是来解决这个问题的。将 <code class="word">comments</code> 字段类型设置为 <code class="literal">nested</code> 而不是 <code class="literal">object</code> 后，<b>每一个嵌套对象都会被索引为一个 <em>隐藏的独立文档</em></b>，比如：</p>
<div class="pre_wrapper lang-json">
<pre class="programlisting prettyprint lang-json">{ <a id="CO269-1"></a><i class="conum" data-value="1"></i>
  "comments.name":    [ john, smith ],
  "comments.comment": [ article, great ],
  "comments.age":     [ 28 ],
  "comments.stars":   [ 4 ],
  "comments.date":    [ 2014-09-01 ]
}
{ <a id="CO269-2"></a><i class="conum" data-value="2"></i>
  "comments.name":    [ alice, white ],
  "comments.comment": [ like, more, please, this ],
  "comments.age":     [ 31 ],
  "comments.stars":   [ 5 ],
  "comments.date":    [ 2014-10-22 ]
}
{ <a id="CO269-3"></a><i class="conum" data-value="3"></i>
  "title":            [ eggs, nest ],
  "body":             [ making, money, work, your ],
  "tags":             [ cash, shares ]
}</pre>
</div>
<div class="calloutlist">
<table border="0" summary="Callout list">
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<td align="left" valign="top" width="5%">
<p><a href="#CO269-1"><i class="conum" data-value="1"></i></a></p>
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<p>第一个 <code class="literal">嵌套文档</code></p>
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<p><a href="#CO269-2"><i class="conum" data-value="2"></i></a></p>
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<p>第二个 <code class="literal">嵌套文档</code></p>
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<p><a href="#CO269-3"><i class="conum" data-value="3"></i></a></p>
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<p><em>根文档</em> 或者也可称为父文档</p>
</td>
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</div>
<p>在独立索引每一个嵌套对象后，对象中每个字段的相关性得以保留。查询也仅仅匹配有相同嵌套对象的文档。</p>

<p>不仅如此，由于嵌套对象也被索引<span style="color:#666;">(在文档内部)</span>，在查询时将嵌套文档与根文档的联接速度非常快 --​ 几乎与单个文档一样快。</p>

<p>这些额外的嵌套文档是隐藏存储的，我们不能直接获取。如果要<b>增删改一个嵌套对象，必须把整个文档重新索引</b>才可以。值得注意的是，查询的时候返回的是整个文档，而不只是嵌套对象。</p>




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